1. Field of the Invention
The present invention relates to an optical encoder, and more particularly to an optical encoder having a sensor module including a light emitting element and a light receiving element. In addition, the present invention also relates to a moving speed controlling device for controlling the moving speed of a member and an apparatus (such as image forming apparatus), which use an optical encoder. Further, the present invention relates to a method for preparing an optical encoder.
2. Discussion of the Related Art
It is well known that in various conventional apparatus such as image forming apparatus, the moving speed of a moving member is detected with an optical encoder to control the moving speed according to the detection result. Optical encoders are classified into transmission type optical encoders and reflection type optical encoders.
Transmission type optical encoders typically include a sensor module including a light emitting element and a light receiving element; and a shade member (such as pulse code wheels) which moves together with a moving member and which has a predetermined pattern located between the light emitting member and the light receiving member to shade the light emitted by the light emitting element toward the light receiving element.
Reflection type optical encoders typically include a light emitting element for emitting light toward a moving member, and a light receiving element for receiving the light reflected from the moving member.
Transmission type optical encoders are superior to reflection type optical encoders in view of detection precision.
In image forming apparatus (such as copiers, facsimiles, and printers), the moving speed of a transfer belt, which feeds a receiving material such that a toner image formed on an image bearing member is transferred onto a predetermined portion of the receiving material, is detected with a transmission type optical encoder to be controlled at a predetermined speed as disclosed in a published unexamined Japanese patent application No. (hereinafter referred to as JP-A) 2005-134763. However, transmission type optical encoders have a drawback concerning assembling property such that it is not easy to precisely attach a sensor module (for use in a transmission type optical encoder) to an apparatus. Therefore, various proposals for properly attaching a sensor module (or a transmission type optical encoder) have been made.
A conventional method for attaching a sensor module for use in transmission type optical encoders will be explained by reference to FIGS. 1-4.
FIGS. 1-4 illustrate a background transmission type optical encoder.
Referring to FIG. 1, an optical encoder 101 includes a sensor module 102, and a pulse code wheel 103, which serves as a shade member and which moves together with a roller 105 (which is a moving member). The roller 105 has a shaft 106, which is rotatably supported by a frame 104 with a bearing 121 (illustrated in FIG. 4) therebetween. The pulse code wheel 103 is pressed into the shaft 106 to be fixed to the shaft.
Referring to FIG. 4, the sensor module 102 includes a light emitting member 107 and a light receiving member 108. The sensor module 102 has bosses 109 and 110 to be respectively fit into round recessed portions 111 and 112 formed on the frame 104.
As illustrated in FIG. 3, the sensor module 102 has holes 113 and 114, which are located on both the sides of the bosses 109 and 110. When the bosses 109 and 110 are fit into the recessed portions 111 and 112, the holes 113 and 114 are respectively linked with holes 115 and 116 (illustrated in FIG. 2) formed on the frame 104. Rivets 117 and 118 are inserted to the linked holes 113-115 and the other linked holes 114-116, respectively, such that the sensor module 102 is fixed to the frame 104.
As illustrated in FIG. 4, in the state in which the sensor module 102 is fixed to the frame 104, the pulse code wheel 103 enters into a space formed by the light emitting member 107 and the light receiving member 108.
The roller 105 rotatably supports a transfer belt (not shown) whose moving speed is to be controlled. In addition, optical modulation tracks (which are not shown in FIGS. 1-8 and which are similar to patterns 50a illustrated in FIG. 18A) are provided on the pulse code wheel 103, and the optical modulation tracks are detected with the sensor module 102 to determined the moving speed of the roller (i.e., the moving speed of the transfer belt). The moving speed of the transfer belt can be controlled at a predetermined speed by performing feedback controlling.
The sensor module 102 is poor in assembling property. The reason therefor will be explained by reference to FIGS. 2 and 4.
When assembling the sensor module 102, at first, the shaft 106 is fixed to the frame 104 with the bearing 121 therebetween. Next, the pulse code wheel 103 is pressed into the shaft 106 to be fixed, and the bosses. 109 and 110 are fit into the recessed portions 111 and 112, respectively, followed by positioning of the sensor module 102. The thus positioned sensor module 102 is fixed to the frame 104 using the rivets 117 and 118. In this assembling operation, it is not easy to press the pulse code wheel 103 into the shaft 106 while fixing the sensor module 102 to the frame 104 such that the pulse code wheel 103 is properly arranged in the space formed by the light emitting member 107 and the light receiving member 108.
Since the pulse code wheel 103 is typically a thin plate made of a material such as metals, resins and glass, a problem which occurs is that the pulse code wheel is contacted with the sensor module 102 in this assembling operation, there by deforming or damaging the pulse code wheel.
In attempting to easily perform the assembling operation, a method in which at first the pulse code wheel 103 is pressed into the shaft 106 to be positioned (i.e., to be fixed to the shaft), and then sensor module 102 is fixed to the frame 104 can be used. In this assembling operation, it is possible that the sensor module is contacted with the fixed pulse code wheel, thereby deforming or damaging the pulse code wheel.
In attempting to prevent such a wheel deforming/damaging problem, a method in which the distance between the light emitting member 107 and the light receiving member 108 is increased can be used. However, in this case the sensing property of the sensor module 102 deteriorates. Therefore, the method is not preferable.
Therefore, a technique of properly positioning a pulse code wheel in the space formed between a light emitting member and a light receiving member using a jig has been used for assembling a sensor module. This technique has a drawback in that the time needed for assembling the sensor module increases.
FIG. 6 illustrates another background transmission type optical encoder. This encoder has a long recessed portion 119 to be engaged with the bosses 109 and 110 instead of the round recessed portions 111 and 112. This optical encoder also has the same drawback as mentioned above.
FIGS. 7 and 8 illustrate another background transmission type optical encoder, which remedies the drawback.
This optical encoder has a recessed portion 120, which is to be engaged with the bosses 109 and 110 and which extends to the edge of the frame 104. When assembling the encoder, at first the shaft 106 is fixed to the frame 104 with the bearing 121 therebetween, and then the pulse code wheel 103 is pressed into the shaft to be fixed thereto. Thereafter, the sensor module 102 is attached to the frame 104 from a direction perpendicular to the shaft 106 (i.e., from the inserting direction indicated by an arrow in FIG. 8). In this inserting operation, the bosses 109 and 110 are guided by the recessed portion 120, and therefore it is possible to perform the assembling operation by merely attaching the sensor module 102 while pressing the sensor module toward the surface of the frame 104. The inserting operation is completed when the boss 109 is contacted with the end of the recessed portion 120. The sensor module 102 is then fixed to the frame 104 using the rivets 117 and 118. Therefore, the wheel deforming/damaging problem is hardly caused.
Optical encoders having a structure similar to the structure illustrated in FIGS. 7 and 8 are disclosed in JP-As 2002-357457 and 2006-129692.
However, the encoder illustrated in FIGS. 7 and 8 has the following drawback.
Specifically, when an optical encoder having such a structure is set as a speed controlling device in an image forming apparatus to control the moving speed of a transfer belt, fixation of the sensor module to the frame 104 deteriorates after long repeated used due to loosening of the rivets 117 and 118 caused by vibration of the image forming apparatus, etc. In this case, a problem in that the position of the sensor module 102 relatively to the frame 104 changes occurs.
More specifically, right after the image forming apparatus is assembled (i.e., the sensor module 102 is attached to the frame 104), the boss 109 is contacted with the end of the recessed portion 120 and thereby the sensor module 102 is properly positioned. However, after long repeated use, the rivets 117 and 118 deteriorate due to vibration, etc., there by deteriorating fixation of the sensor module to the frame 104, resulting in releasing of the boss 109 from the end of the recessed portion 120. In this case, the position of the light emitting member 107 and the light receiving member 108 relative to the pulse code wheel 103 changes, and there by a problem in that the signals, which the sensor module 102 obtains from the rotated pulse code wheel 103 (i.e., the rotated shaft 106), have an error or the sensor module 102 causes misdetection is caused. Therefore, the function of the sensor module 102 cannot be properly fulfilled, resulting in deterioration of image qualities and failure in the image forming apparatus. In addition, when the sensor module 102 is fixed to the frame using the rivets 117 and 118, the fixing operation has to be performed while pressing the sensor module in the inserting direction, resulting in increase of the assembling time. The same is true for the cases where the sensor module is attached to apparatuses other than image forming apparatuses, or the sensor module is fixed using screws instead of the rivets.
Because of these reasons, a need exists for an optical encoder having a sensor module, which can be easily set to an apparatus without damaging the shade member thereof (such as pulse code wheels) and in which the sensor module can maintain the position thereof even after long repeated use.